Ferritic stainless steel for automotive exhaust system with improved heat resistance and condensate corrosion resistance, and method for manufacturing the same

ABSTRACT

Provided are a ferritic stainless steel for automotive exhaust systems with improved heat resistance and condensate corrosion resistance and a method for manufacturing the same. The ferritic stainless steel according to an exemplary embodiment of the present invention includes a stainless steel base material comprising, in % by weight, C: 0.01% or less, Si: 0.5 to 1.0%, Mn: 0.5% or less, P: 0.035% or less, S: 0.01% or less, Cr: 11 to 18%, N: 0.013% or less, Ti: 0.15 to 0.5%, Sn: 0.03 to 0.5%, and the remainder of Fe and other inevitable impurities, and an Al-plated layer formed on the stainless steel base material, wherein the ferritic stainless steel comprises a plating compound comprising (Al19FeMnSi2)5.31 (Aluminum Iron Manganese Silicide) at an interface between the stainless steel base material and the Al-plated layer.

CROSS-REFERENCE OF RELATED APPLICATIONS

This application is the U.S. National Phase under 35 U.S.C. § 371 ofInternational Patent Application No. PCT/KR2017/013589, filed on Nov.27, 2017, which in turn claims the benefit of Korean Patent ApplicationNo. 10-2016-0169695, filed Dec. 13, 2016, the entire disclosures ofwhich applications are incorporated by reference herein.

TECHNICAL FIELD

The present invention relates to a ferritic stainless steel and a methodfor manufacturing the same, and more particularly, a ferritic stainlesssteel for automotive exhaust systems with improved heat resistance andcondensate corrosion resistance and a method for manufacturing the samein order to satisfy characteristics required for the automotive exhaustsystems.

BACKGROUND ART

A cold-rolled stainless steel product, particularly a cold-rolledferritic stainless steel product, has excellent high temperatureproperties such as a thermal expansion rate, a thermal fatigue property,etc., and is resistant to stress corrosion cracking. Therefore, theferritic stainless steel has been widely used in parts for automotiveexhaust systems, home apparatuses, structures, home appliances,elevators, etc.

Generally, members of the automotive exhaust systems are classified intohot parts and cold parts according to the temperature of an exhaust gas.The hot parts of an automobile include an exhaust manifold, a converter,and a bellows, and such parts are generally used at 600° C. or above andhave excellent high temperature strength, thermal fatigue and saltcorrosion properties. On the other hand, the cold parts are members thatare used at approximately 400° C., and include a muffler which reducesnoise caused by an automotive exhaust gas. Due to a condensate corrosionproperty caused by a sulfur (S) component in automotive fuel and anexternal surface rusting corrosion property caused by the use of deicingsalt in the winter, materials such as stainless (or STS) 409, 409L, 439,436L and Al-plated stainless 409 have been used in the cold parts of theautomotive exhaust systems.

For example, STS 409L, the cheapest material among stainless steels, isa type of steel which includes approximately 11 wt % Cr and in which Cand N are stabilized by Ti, thereby preventing sensitivity of a weldedarea and exhibiting excellent processability, is generally used at 700°C. or below, and has some resistance to corrosion caused by a condensatecomponent generated in the automotive exhaust systems, and thus has beenmost widely used.

In a corrosive environment requiring high corrosion resistance, STS 439steels containing 17 wt % Cr, and STS 436L steel containingapproximately 1 wt % Mo as well as STS 439 steel have been used, butthere is a problem of increased cost for parts.

Recently, in countries such as China and India, and in Latin Americancountries, in which automobile production and penetration rate isdrastically increasing in contrast to other developed countries, thesulfur (S) content in gasoline is considerably high. For example, Koreaand Japan regulate the sulfur (S) content in gasoline to be below 10ppm, and China regulates the sulfur (S) content to be below 500 ppm, butgasoline in China actually contains a sulfur content higher than 500 ppmaccording to region.

The sulfur (S) content in gasoline is concentrated by a sulfate ion (SO₄²⁻) of a condensate component in automobile exhaust gas, and a sulfate(H₂SO₄) atmosphere with a high corrosive property of pH 2 or less iscreated. For this reason, corrosion resistance may not be ensured withconventional STS 409L, and thus high chromium-based stainless materialscontaining 17 wt % or more of a Cr component or Mo, such as STS 439 and436L, are gradually being used. However, since there is a problem inthat the resource price of the material gradually increases, there is ademand for the development of a stainless material having pittingcorrosion resistance and condensate corrosion resistance, which does notcontain high priced elements such as Cr or Mo, or only contains a verysmall amount of those elements.

(Patent Document 0001) Korean Unexamined Patent Application PublicationNo. 10-2008-0110662

DISCLOSURE Technical Problem

The present invention is directed to providing a ferritic stainlesssteel for an automotive exhaust system to satisfy properties requiredfor the automotive exhaust system, such as heat resistance and corrosionresistance in a condensate atmosphere by containing Sn in the ferriticstainless steel for the automotive exhaust system, allowing Sn to beconcentrated at the surface area of the stainless steel and forming anAl-plated layer on a base material to form a plating compound at aninterface between the base material and the plated layer.

The present invention is also directed to providing a method formanufacturing the ferritic stainless steel for the automotive exhaustsystem.

Technical Solution

A ferritic stainless steel according to an exemplary embodiment of thepresent invention includes a stainless steel base material comprising,in % by weight, C: 0.01% or less, Si: 0.5 to 1.0%, Mn: 0.5% or less, P:0.035% or less, S: 0.01% or less, Cr: 11 to 18%, N: 0.013% or less, Ti:0.15 to 0.5%, Sn: 0.03 to 0.5%, and the remainder of Fe and otherinevitable impurities, and an Al-plated layer formed on the stainlesssteel base material, wherein the ferritic stainless steel comprises aplating compound comprising (Al₁₉FeMnSi₂)_(5.31) (Aluminum IronManganese Silicide) at an interface between the stainless steel basematerial and the Al-plated layer.

In addition, according to an exemplary embodiment of the presentinvention, the plating compound may further include at least oneselected from a group consisting of Al₉FeSi₂ (Aluminum Iron Silicon),Al₃FeSi₂ (Aluminum Iron Silicon), and Al (Aluminum).

In addition, according to an exemplary embodiment of the presentinvention, Sn may be concentrated on a surface of the stainless steelbase material adjacent to the Al-plated layer by 4.5 times or more ascompared to the stainless steel base material.

In addition, according to an exemplary embodiment of the presentinvention, Sn may be concentrated on the surface of the stainless steelbase material by 4.5 to 6.1 times as compared to the stainless steelbase material.

In addition, according to an exemplary embodiment of the presentinvention, the ferritic stainless steel may include Sn: 0.05 to 0.5%.

In addition, according to an exemplary embodiment of the presentinvention, the ferritic stainless steel may have a maximum pittingcorrosion depth of less than 0.4 mm in the evaluation of condensatecorrosion characteristics (JASO-B M611-92).

In addition, according to an exemplary embodiment of the presentinvention, a chrominance (ΔE) of a surface of the stainless steel beforeand after a heat treatment is 10 or less.

A method for manufacturing a ferritic stainless steel according to anexemplary embodiment of the present invention includes hot-rolling aferritic stainless steel slab comprising, in % by weight, C: 0.01% orless, Si: 0.5 to 1.0%, Mn: 0.5% or less, P: 0.035% or less, S: 0.01% orless, Cr: 11 to 18%, N: 0.013% or less, Ti: 0.15 to 0.5%, Sn: 0.03 to0.5%, and the remainder of Fe and other inevitable impurities;cold-rolling a hot-rolled steel plate; and aluminum-plating acold-rolled steel plate.

In addition, according to an exemplary embodiment of the presentinvention, the ferritic stainless steel is manufactured by aconventional STS 409L manufacturing process.

In addition, according to an exemplary embodiment of the presentinvention, the method may further include heat-treating thealuminum-plated ferritic stainless steel at a temperature of 300 to 500°C. within 48 hours, wherein a chrominance (ΔE) of a surface of thestainless steel before and after the heat treatment is 10 or less.

Advantageous Effects

According to exemplary embodiments of the present invention, a ferriticstainless steel having excellent pitting corrosion resistance andcondensate corrosion resistance without an increase in production costand without a decrease in processability may be manufactured by addingapproximately 0.05 wt % or more of Sn to a pre-existing 11Cr stainlesssteel such as STS 409 among conventional ferritic stainless steels.

In addition, when the ferritic stainless steel according to exemplaryembodiments of the present invention is used in the end part of anexhaust system, for example, in a muffler-related material for anautomotive exhaust system, a member of the automotive exhaust system,which ensures excellent corrosion resistance, without an increase inproduction cost in regions such as China, which uses conventional highsulfur fuel, may be manufactured.

DESCRIPTION OF DRAWINGS

FIG. 1 is a graph showing a maximum pitting corrosion depth of aSn-added stainless steel of the present invention and a Sn-freestainless steel in a condensate solution of an automotive exhaustsystem.

FIG. 2 is a drawing showing the results of analysis of an interfacebetween an Al-plated layer of the Sn-added stainless steel of thepresent invention and a stainless steel base material by transmissionelectron microscopy (TEM) and EDS (Energy-Dispersive Spectroscopy).

FIG. 3 is a drawing showing the results of line analysis by transmissionelectron microscopy (TEM) and EDS (Energy-Dispersive Spectroscopy) fromthe Al-plated layer of the Sn-added stainless steel of the presentinvention to a base material.

FIG. 4 is a graph showing the results of X-ray diffraction (XRD)analysis of the interface between the Al-plated layer of the Sn-addedstainless steel of the present invention and the stainless steel basematerial.

FIG. 5 is a drawing showing the results of the line analysis bytransmission electron microscopy (TEM) and EDS (Energy-DispersiveSpectroscopy) from the Al-plated layer of the Sn-free stainless steel tothe base material.

FIG. 6 is a graph showing the results of the X-ray diffraction (XRD)analysis of the interface between the Al-plated layer of the Sn-freestainless steel and the stainless steel base material.

FIGS. 7 and 8 are photographs showing the Sn-added stainless steel ofthe present invention and the Sn-free stainless steel before and after aheat treatment

MODES OF THE INVENTION

A ferritic stainless steel for automotive exhaust systems with improvedheat resistance and condensate corrosion resistance according to anexemplary embodiment of the present invention includes a stainless steelbase material comprising, in % by weight, C: 0.01% or less, Si: 0.5 to1.0%, Mn: 0.5% or less, P: 0.035% or less, S: 0.01% or less, Cr: 11 to18%, N: 0.013% or less, Ti: 0.15 to 0.5%, Sn: 0.03 to 0.5%, and theremainder of Fe and other inevitable impurities, and an Al-plated layerformed on the stainless steel base material, wherein the ferriticstainless steel comprises a plating compound comprising(Al₁₉FeMnSi₂)_(5.31) (Aluminum Iron Manganese Silicide) at an interfacebetween the stainless steel base material and the Al-plated layer.

Hereinafter, exemplary embodiments of the present invention will bedescribed in detail with reference to accompanying drawings. Thefollowing examples are provided to fully deliver the spirit of thepresent invention to those of ordinary skill in the art. The presentinvention may be specified in different forms without limitation toexamples, which will not be described herein. To clarify the presentinvention, illustration of parts that are not associated with theexplanation will be omitted, and to help in understanding, the sizes ofcomponents will be slightly exaggerated.

The ferritic stainless steel for automotive exhaust systems according toan exemplary embodiment of the present invention comprises, in % byweight, C: 0.01% or less, Si: 0.5 to 1.0%, Mn: 0.5% or less, P: 0.035%or less, S: 0.01% or less, Cr: 11 to 18%, N: 0.013% or less, Ti: 0.15 to0.5%, Sn: 0.03 to 0.5%, and the remainder of Fe and other inevitableimpurities.

In the case of C and N being present in an interstitial form as Ti(C, N)carbonitride-forming elements, Ti(C, N) carbonitride is not formed whenC and N contents are high, and C and N present at a high concentrationdeteriorate elongation and low-temperature impact properties of thematerial. When the material is used at 600° C. or below for a longperiod of time after welding, intergranular corrosion occurs due togeneration of Cr₂₃C₆ carbide, and therefore the content of C ispreferably controlled to be 0.01 wt % or less, and the content of N ispreferably controlled to be 0.01 wt % or less.

Moreover, when the C+N content is high, a number of surface defects suchas scabs may occur due to an increase in steelmaking inclusions causedby adding a high content of Ti, a nozzle blocking phenomenon occurs in acontinuous casting process, and elongation impact properties aredegraded due to an increase in the high contents of C and N. For thisreason, the C+N content is preferably controlled to be 0.02 wt % orless.

Si is an element added as a deoxidizing element, and when its content isincreased as a ferrite-phase forming element, ferrite-phase stability isincreased. When the Si content is increased, a pitting corrosionpotential is increased, and oxidation resistance is increased. In thepresent invention, for the purpose of improvement in pitting corrosionpotential and oxidation resistance, at least 0.5 wt % or more of Si ispreferably contained. If the Si content is increased to 1.0 wt % ormore, steelmaking Si inclusions are increased and surface defects occur.For this reason, the Si content is preferably controlled not to exceed1.0 wt % or more.

When the Mn content is increased, pitting corrosion resistance isdecreased by the formation of a precipitate such as MnS. However,excessive Mn reduction leads to an increase in refining cost, andtherefore, the Mn content is preferably controlled to be 0.5 wt % orless.

Since P and S form grain boundary segregation and an MnS precipitate,leading to degradation in hot workability, it is preferable that a smallamount of P and S are present. However, since excessive reduction leadsto an increase in refining cost, P is preferably controlled to be 0.035wt % or less, and S is preferably controlled to be 0.01 wt % or less.

Cr is an essential element for ensuring corrosion resistance of astainless steel. When the Cr content is low, corrosion resistance isdegraded in a condensate atmosphere, and when the Cr content is toohigh, corrosion resistance is improved, but due to high strength,degradation in elongation and impact properties, and an increase inproduction cost, the Cr content is preferably controlled to be 10 to 18wt %.

Ti is an effective element that fixes C and N to prevent intergranularcorrosion. However, when a Ti/(C+N) ratio is decreased, due tointergranular corrosion occurring at welded areas, corrosion resistanceis decreased, and therefore Ti is preferably controlled to be at least0.15 wt % or more. However, when the Ti content is too high, steelmakinginclusions are increased, a number of surface defects such as scabs mayoccur due to an increase in steelmaking inclusions, a nozzle blockingphenomenon occurs in a continuous casting process, and elongation andimpact properties are degraded. For this reason, the Ti content ispreferably controlled to be 0.5 wt % or less.

Sn is an essential element to ensure heat resistance and corrosionresistance in a condensate atmosphere, for which the present inventionaims.

In the present invention, to ensure heat resistance and condensatecorrosion resistance, Sn is preferably controlled to be at least 0.03 wt% or more. However, since excessive addition of Sn leads to degradationof the manufacturing process, the Sn content is preferably controlled tobe 0.5 wt %. More preferably, the Sn content is controlled to be 0.05 to0.5 wt %. The ferritic stainless steel according to one embodiment ofthe present invention may be an Al-plated ferritic stainless steel,which includes a stainless steel base material and an Al-plated layerformed on the stainless steel base material.

The ferritic stainless steel according to an embodiment of the presentinvention includes (Al₁₉FeMnSi₂)_(5.31) (Aluminum Iron ManganeseSilicide) as a plating compound at the interface between the stainlesssteel base material and the Al-plated layer.

The ferritic stainless steel according to an embodiment of the presentinvention does not include Al₁₇(Fe₃₂Mn_(0.8))Si₂ (Aluminum IronManganese Silicon) as a plating compound at the interface between thestainless steel base material and the Al-plated layer. This is a platingcompound to be formed on Sn-free Al-plated stainless steel, that is, thecomposition of the plating compound formed on the interface between thestainless steel base material and the Al-plated layer differs dependingon whether Sn is added or not.

For example, the plating compound may further include at least oneselected from a group consisting of Al₉FeSi₂ (Aluminum Iron Silicon),Al₃FeSi₂ (Aluminum Iron Silicon), Al (Aluminum).

In the ferritic stainless steel according to the embodiment of thepresent invention, Sn is concentrated on a surface of the stainlesssteel base material adjacent to the Al-plated layer by 4.5 times or moreas compared with the stainless steel base material. Sn is relativelystrong in oxygen affinity compared with other elements, and may beconcentrated on the surface of the stainless steel on which theoxidation scale is formed.

For example, the Sn may be concentrated on the surface of the stainlesssteel base material by 4.5 to 6.1 times as compared to the stainlesssteel base material.

The stainless steel not only includes a region where Sn is concentratedon the surface but also includes (Al₁₉FeMnSi₂)_(5.31) (Aluminum IronManganese Silicide) as a plating compound at the interface between thestainless steel base material and the Al-plated layer, accordingly thedesired improved heat resistance and condensate corrosion resistance maybe obtained as compared with the ferritic stainless steels which do notcontain Sn or in which Sn is not concentrated on the surface.

FIG. 1 is a graph showing a maximum pitting corrosion depth of aSn-added stainless steel of the present invention and a Sn-freestainless steel in a condensate solution of an automotive exhaustsystem.

The embodiment of FIG. 1 is a ferritic stainless steel to which Sn isadded according to an embodiment of the present invention, and acomparative example shows the maximum pitting corrosion depth after theevaluation of condensate corrosion characteristics (JASO-B M611-92) of aconventional Al-coated 409L steel.

For example, the ferritic stainless steel according to one embodiment ofthe present invention may have the maximum pitting corrosion depth ofless than 0.4 mm when evaluating the condensate corrosioncharacteristics (JASO-B M611-92). In contrast, conventional Al-coated409L steels exhibit the maximum pitting corrosion depth of greater thanabout 0.6 mm.

For example, the ferritic stainless steel according to one embodiment ofthe present invention may have a chrominance (ΔE) of the surface of thestainless steel before and after a heat treatment of 10 or less.

The method for manufacturing the ferritic stainless steel according toone embodiment of the present invention may include subjecting aferritic stainless steel slab containing the above composition tohot-rolling, hot annealing, hot pickling, cold-rolling and finishingannealing. This manufacturing process may be a conventional STS 409Lmanufacturing process. Thereafter, the cold-rolled steel sheet may besubjected to an aluminum plating process to produce an Al-coated ferritestainless steel.

For example, the method for manufacturing the ferritic stainless steelaccording to an embodiment of the present invention may further includeheat-treating the Al-coated ferritic stainless steel at a temperature of300 to 500° C. within 48 hours, and the chrominance (ΔE) of the surfaceof the stainless steel before and after the heat treatment is 10 orless.

Hereinafter, the ferritic stainless steel for an automotive exhaustsystem according to an exemplary embodiment of the present invention isdescribed in further detail with reference to examples.

EXAMPLES

Inventive Steel 1

A ferritic stainless steel slab was prepared having the compositionshown in Table 1 below. The slab was hot-rolled at a temperature of1,150° C. to prepare a hot-rolled steel plate having a thickness of 3.0mm. The hot-rolled steel plate was annealed, pickled and cold-rolled toproduce a cold-rolled steel plate having a thickness of 1.2 mm, followedby finish annealing and pickling, and then subjected to Al plating toproduce a final Al-plated ferritic stainless steel product.

Inventive Steel 2

A ferritic stainless steel product was manufactured by the same methodas that described for Inventive Steel 1, except that the composition ofInventive Steel 2 shown in Table 1 below was used.

Inventive Steel 3

A ferritic stainless steel product was manufactured by the same methodas that described for Inventive Steel 1, except that the composition ofInventive Steel 3 shown in Table 1 below was used.

Comparative Steel 1

A ferritic stainless steel product was manufactured by the same methodas that described for Inventive Steel 1, except that the composition ofComparative Steel 1 shown in Table 1 below was used.

Comparative Steel 2

A ferritic stainless steel product was manufactured by the same methodas that described for Inventive Steel 1, except that the composition ofComparative Steel 2 shown in Table 1 below was used.

TABLE 1 Classification C Si Mn P S Cr Ti Sb N Ti/(C + N) Inventive 0.0050.597 0.30 0.021 <0.003 11.14 0.22 0.048 0.0074 17.4 Steel 1 Inventive0.005 0.613 0.31 0.023 <0.003 11.21 0.21 0.11 0.0089 15.1 Steel 2Inventive 0.006 0.592 0.30 0.019 <0.003 11.24 0.24 0.2 0.0072 18.2 Steel3 Comparative 0.005 0.62 0.30 0.023 <0.003 11.24 0.22 0 0.0074 17.7Steel 1 Comparative 0.006 0.594 0.30 0.020 <0.003 11.29 0.23 0.02 0.007218.8 Steel 2

Table 2 shows the results of measurement of main components in theconcentrated layer of the surface of the stainless steel base materialadjacent to the stainless steel base material and the aluminum platinglayer of Inventive Steel 3 above.

TABLE 2 Concentrated Concentrated Concentrated Base layer layer layermaterial 1 (wt %) 2 (wt %) 3 (wt %) (wt %) Fe 79.11074 83.80825 82.1158988.5022  Cr 11.32846  9.647609 10.79504 10.30857  Al  6.784723  4.033176 4.799919  0.157073 Si  1.798595  1.204887  1.276994  0.781254 Sn 0.977485  1.306278  1.012158  0.214902 Concentrated  4.5  6.1  4.7 —layer Sn/Base material Sn

FIG. 2 is a drawing showing the results of analysis of an interfacebetween an Al-plated layer of the Sn-added stainless steel of thepresent invention and a stainless steel base material by transmissionelectron microscopy (TEM) and EDS (Energy-Dispersive Spectroscopy).

Referring to FIG. 2 and Table 2, in the ferritic stainless steelaccording to the embodiment of the present invention, Sn is concentratedon the surface of the stainless steel base material adjacent to theAl-plated layer by 4.5 to 6.1 times as compared to the stainless steelbase material.

FIG. 3 is a drawing showing the results of line analysis by transmissionelectron microscopy (TEM) and EDS (Energy-Dispersive Spectroscopy) fromthe Al-plated layer of the Sn-added stainless steel of the presentinvention to a base material. FIG. 4 is a graph showing the results ofX-ray diffraction (XRD) analysis of the interface between the Al-platedlayer of the Sn-added stainless steel of the present invention and thestainless steel base material.

Referring to FIG. 3, when Cr and Sn were measured from the plated layertoward the base material, it was confirmed that Sn was concentrated onthe surface of the stainless steel base material adjacent to theAl-plated layer.

Referring to FIG. 4, the ferritic stainless steel comprises a platingcompound comprising (Al₁₉FeMnSi₂)_(5.31) (Aluminum Iron ManganeseSilicide), Al₉FeSi₂ (Aluminum Iron Silicon), Al₃FeSi₂ (Aluminum IronSilicon), and Al (Aluminum) at the interface between the stainless steelbase material and the Al-plated layer.

FIG. 5 is a drawing showing the results of the line analysis bytransmission electron microscopy (TEM) and EDS (Energy-DispersiveSpectroscopy) from the Al-plated layer of the Sn-free stainless steel tothe base material. FIG. 6 is a graph showing the results of the X-raydiffraction (XRD) analysis of the interface between the Al-plated layerof the Sn-free stainless steel and the stainless steel base material.

Referring to FIG. 5, when Cr and Sn were measured from the plated layertoward the base material of the Sn-free stainless steel, it wasconfirmed that Sn was not concentrated on the surface of the stainlesssteel base material.

Referring to FIG. 6, the ferritic stainless steel comprises a platingcompound comprising Al₁₇(Fe₃₂Mn_(0.8))Si₂ (Aluminum Iron ManganeseSilicon), Al₉FeSi₂ (Aluminum Iron Silicon), Al₃FeSi₂ (Aluminum IronSilicon), and Al (Aluminum) at the interface between the stainless steelbase material and the Al-plated layer. That is, it can be confirmed thatthe plating compound of (Al₁₉FeMnSi₂)_(5.31) (Aluminum Iron ManganeseSilicide) formed in the Sn-added steel is not formed.

Tables 3 to 5 show the results of measurement of whiteness before andafter a heat treatment of the stainless steel of Inventive Steel 3 towhich Sn was added and the stainless steel of Comparative Steel 1 towhich no Sn was added.

Experimental Example 1

The stainless steel of Inventive Steel 3 and the stainless steel ofComparative Steel 1 were heat-treated at 350° C. for 24 hours, and thewhiteness before and after the heat treatment was measured and is shownin Table 3.

TABLE 3 Whiteness Whiteness before heat after heat treatment treatmentChrominance(ΔE) Inventive Steel 3 77.84 78.04 7.11 Comparative Steel 178.27 71.52 10.83

Experimental Example 2

The stainless steel of Inventive Steel 3 and the stainless steel ofComparative Steel 1 were heat-treated at 400° C. for 24 hours, and thewhiteness before and after the heat treatment was measured and is shownin Table 4.

TABLE 4 Whiteness Whiteness before heat after heat treatment treatmentChrominance(ΔE) Inventive Steel 3 77.93 80.04 5.70 Comparative Steel 177.27 68.97 13.59

Experimental Example 3

The stainless steel of Inventive Steel 3 and the stainless steel ofComparative Steel 1 were heat-treated at 450° C. for 24 hours, and thewhiteness before and after the heat treatment was measured and is shownin Table 5.

TABLE 5 Whiteness Whiteness before heat after heat treatment treatmentChrominance(ΔE) Inventive Steel 3 78.95 79.50 5.38 Comparative Steel 177.59 65.80 15.35

FIGS. 7 and 8 are photographs showing the Sn-added stainless steel ofthe present invention and the Sn-free stainless steel before and after aheat treatment.

FIG. 7 is a photograph of the surface of the stainless steel accordingto Experimental Example 2, and FIG. 8 is a photograph of the surface ofthe stainless steel according to Experiment Example 3.

Referring to the Experimental Examples, FIGS. 7 and 8, the color of thesurface of the steel becomes darker and the whiteness is lowered afterthe heat treatment in the case of the Sn-free steel, but the whitenessdoes not decrease even when the heat treatment is performed in the caseof Sn-added steel, for example, the difference in whiteness is 10 orless. As a result, it can be confirmed that the Sn-added steel issuperior in heat resistance to the Sn-free steel.

The corrosion resistance was evaluated according to Japanese standardJASO-B M611-92 for condensate corrosion resistance.

That is, the maximum pitting corrosion depth was measured afterrepeating a cycle 4 times, and the cycle includes holding the mixture at80° C. for 24 hours in an aqueous solution having a Cl⁻ concentration:100 ppm, a NO₃ ⁻ concentration: 20 ppm, a SO₃ ²⁻ concentration: 600 ppm,a SO₄ ²⁻ concentration: 600 ppm, a CH₃COO⁻ concentration: 800 ppm, and apH of 8.0±0.2 for 5 times, and then holding the mixture at 250° C. for24 hours.

That is, the solution was maintained at 80° C. for 24 hours in anaqueous solution having a Cl⁻ concentration: 100 ppm, a NO₃ ⁻concentration: 20 ppm, a SO₃ ²⁻ concentration: 600 ppm, a SO₄ ²⁻concentration: 600 ppm, a CH₃COO⁻ concentration: 800 ppm, and a pH of8.0±0.2. The maximum formal depth was measured after repeating a totalof 4 cycles with one cycle maintained at 250° C. for 24 hours.

The corrosion resistance of the stainless steels according to InventiveSteel 3 and Comparative Steel 1 was evaluated according to the abovemethod, and the maximum pitting corrosion depth was measured and isshown in Table 6.

TABLE 6 Maximum pitting corrosion depth (mm) Inventive Steel 3 0.35Comparative Steel 1 0.66

FIG. 1 is a graph showing a maximum pitting corrosion depth of aSn-added stainless steel of the present invention and a Sn-freestainless steel in a condensate solution of an automotive exhaustsystem.

The embodiment of FIG. 1 is a Sn-added ferritic stainless steel ofInventive Steel 3, and a comparative example is a conventional Al-plated409L steel of Comparative Steel 1. Referring to FIG. 1 and Table 6,Inventive Steel 3 has a maximum pitting corrosion depth of 0.35 mm andComparative Steel 1 has 0.66 mm when evaluating condensate corrosioncharacteristics (JASO-B M611-92).

As a result, Sn-added Al-plated ferritic stainless steels according toembodiments of the present invention have excellent condensate corrosionresistance and heat resistance.

As described above, while the present invention has been described withreference to exemplary embodiments of the present invention, the presentinvention is not limited thereto, and it will be understood by those ofordinary skill in the art that various modifications and alternationscan be made without departing from the concept and scope of theaccompanying claims.

INDUSTRIAL APPLICABILITY

Ferritic stainless steel according to the embodiments of the presentinvention is excellent in heat resistance and condensate corrosionresistance without adding expensive elements such as Cr and Mo, and maybe applied to an automotive exhaust system.

The invention claimed is:
 1. A ferritic stainless steel for automotiveexhaust systems, comprising: a stainless steel base material comprising,in % by weight, C: 0.01% or less, Si: 0.5 to 1.0%, Mn: 0.5% or less, P:0.035% or less, S: 0.01% or less, Cr: 11 to 18%, N: 0.013% or less, Ti:0.15 to 0.5%, Sn: 0.03 to 0.5%, and the remainder of Fe and otherinevitable impurities, and an Al-plated layer formed on the stainlesssteel base material, wherein the ferritic stainless steel comprises aplating compound comprising Aluminum Iron Manganese Silicide at aninterface between the stainless steel base material and the Al-platedlayer, wherein a chrominance (ΔE) of a surface of the stainless steelbefore and after a heat treatment is 10 or less, wherein a content ofthe Sn is concentrated on a surface of the stainless steel base materialadjacent to the Al-plated layer by 4.5 to 6.1 times as compared to thestainless steel base material.
 2. The ferritic stainless steel accordingto claim 1, wherein the plating compound further comprises at least oneselected from a group consisting of Al₉FeSi₂ (Aluminum Iron Silicon),Al₃FeSi₂ (Aluminum Iron Silicon), and Al (Aluminum).
 3. The ferriticstainless steel according to claim 1, comprising Sn: 0.05 to 0.5%. 4.The ferritic stainless steel according to claim 1, having a maximumpitting corrosion depth of less than 0.4 mm in the evaluation ofcondensate corrosion characteristics (JASO-B M611-92).
 5. A method formanufacturing a ferritic stainless steel for automotive exhaust systemswith improved heat resistance and condensate corrosion resistanceaccording to claim 1, comprising: hot-rolling a ferritic stainless steelslab comprising, in % by weight, C: 0.01% or less, Si: 0.5 to 1.0%, Mn:0.5% or less, P: 0.035% or less, S: 0.01% or less, Cr: 11 to 18%, N:0.013% or less, Ti: 0.15 to 0.5%, Sn: 0.03 to 0.5%, and the remainder ofFe and other inevitable impurities; cold-rolling the hot-rolled steelplate; and aluminum-plating the cold-rolled steel plate.
 6. The methodaccording to claim 5, wherein the ferritic stainless steel ismanufactured by a conventional STS 409L manufacturing process.
 7. Themethod according to claim 5, further comprising heat-treating thealuminum-plated ferritic stainless steel at a temperature of 300 to 500°C. within 48 hours, wherein a chrominance (ΔE) of a surface of thestainless steel before and after the heat treatment is 10 or less.